MONITORING, MODELING AND MITIGATING SUBSIDENCE IN THE LOST HILLS FIELD

Subsidence resulting from hydrocarbon production in the Lost Hills Field is occurring at the rate of approximately nine inches per year. Over the past ten years the ground surface has subsided more than eight feet resulting in casing damage and well failures. Casing damage can occur within two years of drilling a well and often limits the ability to perform workovers or recompletions. Once a well bore fails a costly replacement well is required.

To understand subsidence Chevron has been monitoring, mapping and modeling. One hundred forty-two field monuments are surveyed by GPS every six months to record the amount of subsidence. Radioactive markers were permanently placed in several new wells to determine which zones are suffering the greatest compaction. Casing damage depths have been mapped and correlated to several major slip planes. A geomechanical model was built to simulate reservoir response to various production and injection scenarios.

Prevention of subsidence is more of a challenge than monitoring. A waterflood was established in 1992 and has been effective in slowing the rate of subsidence although well failures still occur.

Recently modifications have been made to the casing design which allow for some movement before damage occurs. By increasing the open hole size and/or the diameter of the casing string it may be possible to delay casing damage thereby extending the life of the well.

Several types of casing remediation have been attempted in an effort to restore damaged wells to production.